Page 27 - Solid State

Applications of n-type and p-type semiconductors
Various combinations of n-type and p-type semiconductors are used
for making electronic components. Diode is a combination of n-type
and p-type semiconductors and is used as a rectifier. Transistors are
made by sandwiching a layer of one type of semiconductor between
two layers of the other type of semiconductor. npn and pnp type of
transistors are used to detect or amplify radio or audio signals. The
solar cell is an efficient photo-diode used for conversion of light energy
into electrical energy.
Germanium and silicon are group 14 elements and therefore, have
a characteristic valence of four and form four bonds as in diamond. A
large variety of solid state materials have been prepared by combination
of groups 13 and 15 or 12 and 16 to simulate average valence of four
as in Ge or Si. Typical compounds of groups 13 – 15 are InSb, AlP
and GaAs. Gallium arsenide (GaAs) semiconductors have very fast
response and have revolutionised the design of semiconductor devices.
ZnS, CdS, CdSe and HgTe are examples of groups 12 – 16 compounds.
In these compounds, the bonds are not perfectly covalent and the ionic
character depends on the electronegativities of the two elements.
It is interesting to learn that transition metal oxides show marked
differences in electrical properties. TiO, CrO 2 and ReO 3 behave like
metals. Rhenium oxide, ReO 3 is like metallic copper in its conductivity
and appearance. Certain other oxides like VO, VO 2 , VO 3 and TiO 3
show metallic or insulating properties depending on temperature.
1.11 Magnetic
Properties Every substance has some magnetic properties associated with it.
The origin of these properties lies in the electrons. Each electron in an
atom behaves like a tiny magnet. Its magnetic moment originates
from two types of motions (i) its orbital
motion around the nucleus and (ii) its spin
around its own axis (Fig. 1.31). Electron
being a charged particle and undergoing
these motions can be considered as a small
loop of current which possesses a magnetic
moment. Thus, each electron has a
permanent spin and an orbital magnetic
moment associated with it. Magnitude of
Fig.1.31: Demonstration of the magnetic moment this magnetic moment is very small and
associated with (a) an orbiting electron is measured in the unit called Bohr
and (b) a spinning electron. –24 2
magneton, μ B . It is equal to 9.27 × 10 A m .
On the basis of their magnetic properties, substances can be
classified into five categories: (i) paramagnetic (ii) diamagnetic (iii)
ferromagnetic (iv) antiferromagnetic and (v) ferrimagnetic.
(i) Paramagnetism: Paramagnetic substances are weakly attracted
by a magnetic field. They are magnetised in a magnetic field in
the same direction. They lose their magnetism in the absence of
magnetic field. Paramagnetism is due to presence of one or
more unpaired electrons which are attracted by the magnetic
3+
3+
2+
field. O 2 , Cu , Fe , Cr are some examples of such substances.
27 The Solid State